Why is Q so facinated by the Borg?

Maybe because they waste nothing of value. They use it all to enhance the collective.

Perhaps he sees them as the only species that is advancing rapidly enough to be worthy of notice. Or maybe he's amused by how the mortals' own technology is overtaking them and turning them into something other than themselves, like some kind of cruel irony. Or maybe he sees them as a useful tool with which to test the mortals, to see if they can overcome the threat.

He means they're the only species that don't get distracted by love, family, career, ambition, sex, art, self-awareness, pleasure, pain etc.

They simply consume as one and discard what is superfluous.

For all we know, he created the Borg somehow.